Cocoa carbon classic and compatibility

Kelly McNeill writes "Mac OS X is an astounding piece of work. It is built atop a solid UNIX foundation, installs effortlessly, and is almost as easy to use as the original Mac OS.

The OS also gives users the freedom to run software built for Mac OS 9 and below inside a somewhat invisible classic environment, while offering developers a choice between modifying their existing programs to run natively inside the OS's Carbon layer or easily writing programs from scratch within the powerful Cocoa APIs developed at NeXT. However, there still remains a considerable number of Mac users who are hesitant to switch to Apple's shiny new operating system."